The last heat face of a steam boiler before the smoke stack is either a flue-gas/air heat exchanger or an economizer. In the present application, a flue-gas/air heat exchanger is understood as a heat exchanger between flue gas and combustion air, in which the heat is transferred from the flue gas into the combustion air to preheat the combustion air. In the present application, an economizer is understood as a heat exchanger in which thermal energy is transferred from the flue gases into the supply water.
When a flue-gas/air heat exchanger is used, the supply water for the boiler can be preheated by means of bled steam from a steam turbine, whereby the efficiency of the steam turbine process is enhanced. A flue-gas/air heat exchanger, i.e. a heat exchanger, in which thermal energy is transferred from the flue gases directly into the combustion air is not usually used in small steam power plants because of its high cost.
When a flue-gas/air heat exchanger is not used, the flue gases of the steam boiler are cooled with the aid of an economizer before passing into the smoke stack. In such case, the supply water cannot be preheated with the aid of bled steam of the steam boiler because the preheating would raise the ultimate temperature of the flue gases and thereby, impair the efficiency of the boiler.
In an economizer of a steam boiler, heat is transferred from the flue gases into the supply water. A steam boiler provided with a combustion chamber is used as the steam boiler. A change in the temperature of the supply water in the economizer is lower than a change in the temperature on the flue-gas side. A temperature rise in the supply water is usually 40 to 50 per cent of the respective the temperature drop on the flue-gas side. Hence, a difference of temperature on the hot end of the economizer is considerably higher than on the cold end. A result of this observation is that, in addition to the heat obtained from the flue gases, different kind of heat can be transferred into the supply water. In a steam turbine process, it is advantageous to utilize bled steam for preheating the supply water.
The economizer of the steam boiler in a steam power plant is divided into two or more parts, the supply water being preheated in the preheaters of the high-pressure side provided between said economizer parts by the bled steam from the steam turbine.
With the aid of a connection, the integration of the steam boiler and the steam turbine process is made more efficient. By means of such arrangement, the flue gases of the steam boiler can be cooled efficiently, and simultaneously enhancing the efficiency of the steam turbine process.
The investment cost is lower than in an alternative provided with a flue-gas/air heat exchanger:                improved controllability and boiler efficiency        smaller boiler building        lower cost of the boiler.        
When a flue-gas/air heat-exchanger solution is unprofitable, an improved process can be implemented with the structure since the use of bled steam can be increased.
The arrangement is preferred especially in an instance in which the combustion air of the steam boiler is heated in one or more steam/air heat exchanger(s) connected in series and utilizing bled steam.
In a prior FI patent No. 101 163, which corresponds to EP 0724683, of the applicant, the advantageous integration construction between the steam boiler and the steam turbine is known. It has proved to be useful that the temperature of the supply water flown through the economizers positioned in the flue-gas duct can be controlled. An amendment to the integration construction disclosed in the FI patent No. 101 163 is presented in the present application.